Can giant planets form by direct gravitational instability?

Gravitational instability has been invoked as a possible mechanism of the giant planet production in protoplanetary disks. Here we critically revise its viability by noting that to form planets directly, it is not enough for protoplanetary disks to be gravitationally unstable. They must also be able to cool efficiently (on a timescale comparable to the local disk orbital period) to allow the formation of the bound clumps by fragmentation. A combination of the dynamical and thermal constraints puts very stringent lower limits on the properties of disks capable of fragmenting into the self-gravitating objects: for the gravitational instability to form giant planets at 10 AU in the disk cooled by the radiation transfer, the gas temperature must exceed 103 K with a minimum disk mass of 0.7 M-circle dot and a luminosity of 40 L-circle dot. Although these requirements are relaxed in the more distant parts of the disk, masses of the bound objects formed as a result of instability are too large even at 100 AU (similar to10M(J)) to explain the characteristics of known extrasolar giant planets. Such protoplanetary disks ( and planets formed in them) have very unusual observational properties, and this severely constrains the possibility of giant planet formation by direct gravitational instability.